1. Field of the Invention
The present invention relates to a process for producing a polyol which is useful as a material for polyester resins, alkid resins, polyurethane resins, polycarbonate resins, plasticizers, lubricants, surfactants, base materials for cosmetics, and reactive monomers.
2. Description of the Related Art
In general, a process for producing a polyol includes a step of reaction, a step of extraction of the polyol from the reaction product liquid, a step of separation of the extracting reagent and a step of purification of the product by distillation. In the step of reaction, it is known that an aliphatic aldehyde and formaldehyde can be reacted in two steps comprising the aldol condensation and the cross-Cannizzaro reaction, successively (U.S. Pat. No. 3,935,274 and Japanese Patent Application Laid-Open No. 61(1986)-18741).
The reaction product liquid is concentrated, where necessary, and separated into salts of formic acid and the polyol in the step of extraction in accordance with a conventional process of extraction (Japanese Patent Application Publication No. Showa 52(1977)-30486 and Showa 44(1969)-10767). Then, in the step of separation of the extracting reagent, the extracting reagent is separated from an extract liquid containing the polyol in accordance with a distillation process. The obtained crude polyol is purified in the step of purification by distillation. For example, when the extraction is conducted using a solvent such as an alcohol and a ketone as the extracting reagent and then the extracting agent is separated, 0.5 to 2% of a salt of formic acid remains in the crude polyol. When the salt of formic acid is heated during purification of the crude polyol by distillation, the salt is converted into a basic compound. The formed basic compound triggers heat decomposition of the polyol. Therefore, the crude polyol containing the salt of formic acid cannot be purified by distillation without any treatments. It is generally conducted that the salt of formic acid is deactivated by adding an acid such as phosphoric acid so that the heat decomposition of the polyol is suppressed (Japanese Patent Application Laid-Open No. Showa 63(1988)-139141).
Recently, polyols are used in a wide variety of fields. In particular, when polyols are used as the raw materials for resins curable by ultraviolet light, polyols having a more excellent quality than conventional products are required. However, the quality of polyols produced by distillation in accordance with a batch process fluctuates among fractions. Therefore, to satisfy the requirement for the excellent quality, it is necessary that the quality be stabilized by conducting the distillation in accordance with a continuous process.
As described above, when the reaction product liquid is treated by extraction using an alcohol or a ketone as the extracting reagent in accordance with a conventional process, an acid is added to the reaction product liquid to deactivate a salt of formic acid since 0.5% or more of the salt of formic acid remains in the crude polyol. In this case, when the purification by distillation is conducted in accordance with a continuous process, salts of the acid such as salts of phosphoric acid are precipitated at the inside and at the bottom portion of the distillation column and clogging takes place. Therefore, a stable continuous operation cannot be achieved. Thus, the distillation must be conducted in accordance with the batch operation and fluctuation in the quality of the product is inevitable. Moreover, when the reaction product liquid is treated by extraction with an alcohol or a ketone, the yield of the extracted polyol is small and the cost of production increases. The quality of the salt of formic acid as a byproduct also deteriorates.
When an aliphatic aldehyde is used as the extracting reagent, the yield of the extracted polyol increases and the fraction of the removed salt of formic acid also increases. For example, a process in which butyraldehyde is used as the extracting reagent is described in Japanese Patent Application Publication Heisei 4(1992)-17169. In accordance with this process, the amount of a salt of formic acid remaining in the extracted and separated polyol can be suppressed to 0.3% or less. However, although the continuous distillation of this crude polyol can be conducted, a great amount of acetals are formed from the polyol and the aldehyde used as the extracting reagent in distillation under an atmospheric pressure for separating the extracting reagent since aldehyde which is used as the extracting reagent is very reactive itself. For example, when the aliphatic aldehyde is normal-butyraldehyde (referred to as NBAL, hereinafter) and the polyol is trimethylolpropane (referred to as TMP, hereinafter), TMP-NBAL acetal expressed by formula (iii): is formed as a byproduct. NBAL aldol expressed by formula (iv): is formed as another byproduct by the reaction between NBAL molecules. Moreover, methanol and 2-alkenol expressed by formula (v): which is an intermediate reaction product of methanol and TMP contained in the reaction product liquid is extracted. These compounds are separated and recovered together with the extracting reagent during separation of the extracting reagent.
When distillation is conducted at a low temperature under a reduced pressure, the fraction of the recovered solvent decreases although acetals are not formed as byproducts, and this process cannot be used practically.
When the recovered aldehyde is repeatedly used as the extracting reagent, these impurities are accumulated and adversely affect the extraction. The quality of the polyol is also adversely affected. To overcome this problem, the extracting reagent may be regenerated by distillation. However, the regeneration requires complicated operations and is industrially disadvantageous.
The amount of the salts of formic acid remaining in the extract liquid may be decreased by washing the extract liquid with water after the extraction. In this case, the aqueous layer separated in the step of washing with water contains the polyol in some amount and it is desirable that the washing water is reused.
In recycling the washing water to the step of extraction, when the aqueous layer separated in the step of washing with water is mixed with the concentrated reaction liquid to be treated by the extraction without any treatments and used for the extraction, the concentration of water in the liquid to be treated by the extraction increases and the efficiency of the extraction decreases. The reaction product liquid is therefore concentrated to prevent the decrease in the efficiency of the extraction. However, when the reaction product liquid is concentrated excessively (a concentration of the salt of formic acid of 25% or greater), problems such as clogging of piping due to separation of the salt of formic acid take place and the operation becomes difficult.
On the other hand, when the washing water is recycled to the step of concentration of the reaction product liquid without any treatments, in other words, when the washing water containing the extracting reagent is recycled, degeneration of the extracting reagent and side reactions of the extracting reagent with the polyol take place.